Wafer level chip scale package and process of manufacture

ABSTRACT

Power wafer level chip scale package (CSP) and process of manufacture are enclosed. The power wafer level chip scale package includes all source, gate and drain electrodes located on one side of the device, which is convenient for mounting to a printed circuit board (PCB) with solder paste.

PRIORITY CLAIM

This application is a divisional application of and claims the benefitof priority of commonly assigned U.S. patent application Ser. No.13/007,356, filed Jan. 14, 2011, the entire disclosures of which areincorporated herein by reference. U.S. patent application Ser. No.13/007,356 is a continuation of and claims the priority benefit of U.S.patent application Ser. No. 12/023,921, filed Jan. 31, 2008, the entiredisclosures of which are incorporated herein by reference.

FIELD OF THE INVENTION

This invention generally relates to semiconductor packaging and morespecifically relates to a low cost process of wafer level chip scalepackage (WLCSP).

BACKGROUND OF THE INVENTION

A low package resistance and good thermal performance is often desirablefor semiconductor devices. This is particularly the case for metal oxidesemiconductor field effect transistor (MOSFET) devices, especiallyvertical conduction power MOSFET devices having gate and sourceelectrodes on one surface of a semiconductor chip and a drain electrodeon the opposite surface. It is also generally desirable to have simple,quick and efficient methods of packaging semiconductor devices. Thus,numerous packaging concepts and methods have been developed in the priorart.

While silicon process technology has advanced significantly in the pastdecade, for the most part, the same decades-old packaging technologycontinues as the primary packaging means. Epoxy or solder die attachmentalong with aluminum or gold wire bonding to a lead frame is still thedominant semiconductor packaging methodology. Advances in semiconductorprocessing technology, however, have made parasitics (e.g., resistances,capacitances and inductances) associated with conventional packagingtechniques more of a performance-limiting factor. In the case ofconventional flip chip technology, among other shortcomings, electricalconnection to the back side of the die is not easily facilitated. Theselimitations become quite significant in high current applications suchas power switching devices.

U.S. Pat. No. 6,767,820 discloses a chip scale package of semiconductorMOS-gated device. A source side of a MOS-gated device wafer is coveredwith a passivation layer, preferably a photosensitive liquid epoxy, or asilicon nitride layer, or the like. The material is then dried and thecoated wafer is exposed using standard photolithographic techniques toimage the wafer and openings are formed in the passivation layer toproduce a plurality of spaced exposed surface areas of the underlyingsource metal and a similar opening to expose the underlying gateelectrode of each die on the wafer. The openings in the passivationlayer are typically made through to a conventional underlying solderabletop metal such as titanium, tungsten, nickel, or silver. After theopenings are formed, the wafer is then sawn or otherwise singulated intoindividual die. The solderable drain side of the die is then connectedto a U-shaped or cup-shaped drain clip, using a conductive epoxy orsolder, or the like to bond the bottom drain electrode of the die to thedrain clip. The bottoms of the legs of the drain clip are coplanar withthe source-side surface (that is the tops of the contact projections) ofthe die. U-shaped clip is usually made of a copper alloy with at leastpartially plated silver surfaces and is actually very thin. However,connecting dies to individual clips tends to be time consuming comparedwith wafer level process. In addition, different U-shaped clips aretypically needed for different die sizes, and the clips take extra spaceon the PC board.

US publication number 2003/0052405 discloses a vertical power MOSFETdevice with the drain electrode formed on the bottom surface of thesilicon substrate connected to the lead frame above it whereas the gateelectrode and the source electrode exposed to the bottom of the device.The MOSFET device is sealed by a resin, such as epoxy or silicone, suchthat the MOSFET device and an inner part of the lead frame are covered.On the bottom surface of the MOSFET device, the surface of the resin isapproximately flush with surfaces of the lead frame and gate/sourceelectrodes. That is, on the bottom surface of the semiconductor device,the bottom surface of outer lead portions of the lead frame and bottomsurfaces of gate/source electrodes are exposed for connection to aconductor land (mount surface) of the mounting substrate. Then theperimeter of these gate/source electrodes is covered by the resin.

U.S. Pat. No. 6,133,634 discloses a flip chip package having a powerMOSFET device including a drain terminal, a source terminal and a gateterminal. The drain terminal connects to a conductive carrier and anouter array of solder balls. The source terminal and gate terminalconnect to an inner array of solder balls. The conductive carrier andthe outer array of solder balls provide electrical connection betweenthe printed circuit board and the drain terminal.

U.S. Pat. No. 6,469,384 discloses a method of packaging semiconductordevices, such as MOSFET device, which does not require a molded body.The MOSFET device is coupled to a substrate such that the source andgate regions of the die are coupled to the substrate. The MOSFET deviceis placed on a printed circuit board (PCB) and the surface of the die iscoupled directly to the PCB with solder paste or suitable electricallyconductive interconnect, and thus serves as the drain connection. Thesurface of the die coupled to the substrate comprises the gate regionand the source region of the die. Thus, the solder ball in the gateregion of the substrate serves to couple the gate region of the die tothe PCB while the remaining solder balls couple the source region of thedie through the substrate to the PCB.

The preceding prior art package designs for vertical power MOSFETdevices can provide electrical interconnection for source, gate anddrain for individual MOSFETs. However, additional assembly steps areneeded after a wafer has been singulated into individual dies, whichincreases costs and fabrication time. In addition, the use of metalclips to provide drain contacts from the back to front sides of the diecan reduce the available space for the die on a PCB. It would bedesirable to produce a package design and process for its manufacturewhich permits wafer level processing with lower costs and a reducedfootprint for individual part.

It is within this context that embodiments of the present inventionarise.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and advantages of the invention will become apparent uponreading the following detailed description and upon reference to theaccompanying drawings in which:

FIG. 1A is a perspective view from a front side (source and gate side)of a semiconductor device according to an embodiment of the presentinvention.

FIG. 1B is a perspective view form a back side (drain side) of thesemiconductor device of FIG. 1A.

FIG. 1C is a perspective view of an alternative configuration for thefront side of a semiconductor device according to an embodiment ofpresent invention.

FIGS. 2A-2Q are schematic diagrams showing a process of manufacturing ofwafer level chip scale package of vertical power MOSFET of FIGS. 1A-1Baccording to an embodiment of the present invention.

FIGS. 3A-3I are schematic diagrams showing an alternative process ofmanufacturing of wafer level chip scale package of vertical power MOSFETof FIG. 1C according to another embodiment of the present invention.

FIGS. 4A-4B are schematic diagrams illustrating a method for mounting avertical power MOSFET of the type shown in FIG. 1C to a PC board.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Although the following detailed description contains many specificdetails for the purposes of illustration, anyone of ordinary skill inthe art will appreciate that many variations and alterations to thefollowing details are within the scope of the invention. Accordingly,the examples of embodiments of the invention described below are setforth without any loss of generality to, and without imposinglimitations upon, the claimed invention.

FIGS. 1A-1B are perspective views from the front side and the back sideof a semiconductor device 100 according to a preferred embodiment of thepresent invention. By way of example, the device 100 may be a verticalpower MOSFET. As shown in FIG. 1A, source electrodes (S) 108 and gateelectrode (G) 110 are located at the front side of the device 100,connecting to an underlying source pad and a gate pad through openingwindows on a passivation layer 102, which is deposited on a substrate112 made of a semiconductor material, such as silicon. The source padand gate pad are connected to the source regions and gate regions on thefront side of the vertical power MOSFET with a drain region normallylocated at the backside of the device 100. In the embodiment shown inFIG. 1A, drain electrodes (D) 107 may be located at the trimmed corners106 at the front side of the device 100. The drain electrodes (D) 107are electrically connected to a drain region proximate the backside ofthe device 100 by an electrically conductive layer 104 on the backsideand over the sidewalls 105 of the substrate 112 at the trimmed corners106 as shown in FIGS. 1A-1B. The conductive layer 104 may be formed byselectively electroplating portions of the device 100 with metal, suchas copper, or by electrolessly plating selected portions of the device100 with a metal combination such as nickel/gold (Ni/Au). NiAu comprisesof a layer of nickel with a relatively thin layer of gold on it toprevent oxidation.

The drain electrodes 107 may be configured such that they extend over aportion of active device area 114 of the device covered by passivationat the front side. Such a configuration may minimize the loss of activedevice area and also allow for larger area and lower resistance in thesource electrodes 108 and gate electrodes 110. In some embodiments thedrain electrodes 107 may be omitted and electrical connection to thedrain region may be made through the conductive layer 104 on theside-walls of the trimmed corners 105 as shown in FIG. 1C.

FIGS. 2A-2P are schematic diagrams illustrating an example of a processfor wafer-level chip scale packaging of semiconductor devices (e.g.,vertical power vertical power MOSFETs) of the type described in FIGS.1A-1B. As shown in FIG. 2A, processing may start with a substrate 206with a plurality device structures fabricated on it. By way of example,the substrate 206 may be a semiconductor wafer, such as a silicon wafer,comprising a plurality of semiconductor dies. A passivation layer (notshown) may be deposited on the substrate 206, with gate pads (G) 202 andsource pads (S) 204 exposed through window openings on the passivationlayer at the top surface of the wafer. A metal seed layer 208 isdeposited on selected portions of the top surface of the substrate 206over the gate areas (G) 202 and source areas (S) 204 as shown in FIG.2B. The seed layer 208 may be a thin metal layer or metal alloy layercompatible to the metal material to be deposited over it. By way ofexample, in a case where copper (Cu) is to be used as an electrodematerial, the seed layer 208 may be formed by a layer of TiCu withthickness less than 4 um. Holes 210 are formed in seed layer 208 asshown in FIG. 2C by etching through a mask. As shown in FIG. 2D, photoresist mask 212 is deposited on the seed layer 208. The resist layer 212may be patterned with openings over the gate pads 202 and the sourcepads 204. A thick metal layer 214 may be plated on the seed layer 208 toform gate electrodes 213 and source electrodes 215 as shown in FIG. 2E.By way of example, copper (Cu) may be electrically plated over the seedlayer 208 with thickness larger than 1 um, preferably larger than 10 umat the openings in the resist layer 212. The substrate 206 may then beback grinded to a desired thickness as shown in FIG. 2F, preferably lessthan 400 um.

After back grinding, one or more through holes 211 may be etched throughthe substrate 206 as shown in FIG. 2G, e.g., using photo resist 212 anda thick metal layer 214 as mask. FIG. 2N is a top or bottom view of thewafer with through holes 211 as described in FIG. 2G. The substrate 206may then be isotropically etched, e.g., by an isotropic Si etch followedby oxide etch (wet bench), to form rounded edges 209 of the throughholes 211 at the back side of the substrate 206 as shown in FIG. 2H.This increases mechanical strength and improves uniformity of aconductive layer to be formed on the back surface and inside wall of thethrough holes 211 in subsequent steps. A metal seed layer 216 is formedon the back of the substrate 206 and side-walls of the through the holes211 as shown in FIG. 2I.

A thick layer of metal 218, e.g., Cu may be plated over the seed layer216 as shown in FIG. 2J. Photo resist 212 is then removed follow byetching of the seed layer 208 as shown in FIGS. 2K-2L to form isolatedgate electrodes 217 and source electrodes 219 over the gate pad 202 andsource pad 204 respectively. FIGS. 2O-2P are top and bottom views of thewafer as described in FIG. 2L. As shown in FIG. 2O, drain electrode(s)214 are located at the corners of each MOSFET structure and partiallycover some of the active area 215 of the device. The drain electrodes214 are electrically connected to drain regions proximate the back sideof the substrate 206 by the layer of metal 218 coating the back side andthe side walls of the holes 211.

The wafer may then be diced to form individual devices 220 as shown inFIG. 2M. The dicing process cuts through the holes 211 but leaves aportion of the sidewalls of the holes and a corresponding portion of themetal layer 218 coating the sidewalls, which provide electricalinterconnections between a backside drain region and the drainelectrodes 214.

The through holes 211 are not restricted to circular profiles. Forexample, FIG. 2Q shows an alternative embodiment of the invention inwhich the through holes 211 have a non-circular profile, but instead are“plus sign”-shaped. Other profiles for the through holes 211 are withinthe scope of embodiments of the present invention.

FIGS. 3A-3I are schematic diagrams illustrating an alternative processof manufacturing power wafer level chip scale packaging of semiconductordevices. As shown in FIG. 3A, a wafer, which includes a plurality ofdevice structures (e.g., vertical power MOSFETs) includes asemiconductor substrate 306. A passivation layer (not shown) may bedeposited on the silicon substrate 306 with gate pads (G) 302 and sourcepads (S) 304 exposed through opening windows on the passivation layer atthe top surface of the wafer.

A photo resist mask 308 is deposited on the top surface of the waferover the gate areas (G) 302 and source areas (S) 304 with holes 310located at corner intersections between two or more structures as shownin FIG. 3B. The substrate 306 may then be back grinded to a desiredthickness as shown in FIG. 3C.

The substrate 306 is etched through holes 310 to form through holes 311as shown in FIG. 3D. The silicon 306 is then isotropically etched, e.g.,using a silicon etch followed by an oxide etch (wet bench), to formrounded edges 309 of the holes 311 at the back side as shown in FIG. 3E.A first metal layer 312 is formed on the back side of the substrate 306and through the holes 311 coating the side walls of the holes 311 asshown in FIG. 3F. By way of example, the first metal layer 312 may beany metal suitable for electroless Nickel plating, such as an Al or Alalloy deposited over a thin Ti layer. The combined thickness of thefirst metal layer 312 may be larger than 1 μm, preferably larger than 3μm. After the first metal layer 312 is formed, the photo resist mask 308is then removed as shown in FIG. 3G. A second metal layer 314, such aselectroless Ni/Au, is electrolessly plated over the first metal layer312, as shown in FIG. 3H. By way of example, the Ni thickness may bebetween 1-10 μm and the Au thickness may be less than 1 μm, with a totalcombined thickness less than 11 μm. The process that forms the secondmetal layer 314 may be one in which metal grows on the gate pads 302,source pads 304 and over the metal layer 312 to form gate electrodes313, source electrodes 315 and drain electrodes 317. The wafer isfinally diced to form individual vertical power MOSFET 316 as shown inFIG. 3I.

By contrast, in a conventional process, the front side, which includesgate and source electrodes, is normally plated separately from the backside, so the back side needs to be protected during front side metaldeposition and different metal is used for front and back sides.Normally, an Al—Si—Cu alloy is used on front side. Al is normally notused on back side since it is difficult to solder when mounting to a PCboard. In the forgoing embodiments, by contrast, the same metal may beused for the source, gate and drain connections. This simplifies themanufacturing and reduces the cost.

FIGS. 4A-4B are schematic diagrams showing the steps of mounting a powerwafer level chip package scale vertical power MOSFET of the type in FIG.1C to a printed circuit board (PCB).

As shown in FIG. 4A, a WLCSP device 400 of the type depicted in FIG. 1Chaving a front side gate electrode 402, source electrode 404 and sidewall drain electrode 406 may be bonded to a PC board 401. Solder paste408 may be deposited at the electrodes 410 of the PCB 401 followed byflip-chip mounting the device on to the PCB. The solder paste 408 isthen reflowed to form electrical interconnection between the gateelectrode 402, source electrode 404, drain electrode 406 andcorresponding electrodes 410 of the PCB 401 as shown in FIG. 4B. Thesolder paste 408, after reflow, may also wet the drain metal electrode406 on the side walls at the trimmed corners, which results in a smallresistance.

Embodiments of the present invention avoid the use of metal contacts,such as caps or other structures, or post singulation manufacturingsteps to provide contact between the front and back sides of asemiconductor device chip. Embodiments of the present invention allowelectrical contact to be made between front and back sides of asemiconductor device while the device is still part of a wafer andbefore the wafer is singulated into individual device chips. Embodimentsof the present invention facilitate simple, efficient and cost effectivewafer level chip scale packaging of semiconductor devices.

Although the specification shows a vertical power MOSFET, this inventionis also applicable to any type of vertical semiconductor device, such asan insulated-gate bipolar transistor (IGBT), a bottom source MOSFET, ora bipolar power transistor.

While the above is a complete description of the preferred embodiment ofthe present invention, it is possible to use various alternatives,modifications and equivalents. Therefore, the scope of the presentinvention should be determined not with reference to the abovedescription but should, instead, be determined with reference to theappended claims, along with their full scope of equivalents. Anyfeature, whether preferred or not, may be combined with any otherfeature, whether preferred or not. In the claims that follow, theindefinite article “A”, or “An” refers to a quantity of one or more ofthe item following the article, except where expressly stated otherwise.The appended claims are not to be interpreted as includingmeans-plus-function limitations, unless such a limitation is explicitlyrecited in a given claim using the phrase “means for.”

What is claimed is:
 1. A semiconductor device comprising: a semiconductor substrate having a first pad and a second pad respectively electrically connected to first and second regions located on a front side of the substrate; a first electrode electrically connected to the first pad; a second electrode electrically connected to the second pad; and a third electrode comprising a conductive layer on a side wall of a trimmed corner extending to the back side of the substrate, wherein the conductive layer is electrically connected to a third region located at a back side of the substrate.
 2. The semiconductor device of claim 1, wherein the device comprises a vertical power MOSFET.
 3. The semiconductor device of claim 2, wherein the first region comprises a source region; the second region comprises a gate region, and the third region comprises a drain region.
 4. The device of claim 3, wherein the source and gate regions are located proximate the front side and the drain region is located proximate the back side.
 5. The semiconductor device of claim 1 wherein the first, second and third electrodes are made from the same conductive material.
 6. The semiconductor device of claim 1, wherein the first, second and third electrodes are made from conductive materials selected from the group consisting of copper or electroless Ni/Au layers.
 7. The semiconductor device of claim 1 wherein said third electrode further comprising a conductive layer extending onto a portion of the front side of the substrate.
 8. The semiconductor device of claim 7 wherein the third electrode is configured such that the conductive layer extends over a portion an active region of the substrate on the front side.
 9. The semiconductor device of claim 1 wherein the device is configured to be mounted on a printed circuit board such that electrical connection from the printed circuit board to the first, second and third electrodes are facing and in electrical contact with corresponding connections on the printed circuit board. 